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The Generating Mechanisms of the August 17, 1999 I˙Zmit Bay (Turkey) Tsunami: Regional (Tectonic) and Local (Mass Instabilities) Causes

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The Generating Mechanisms of the August 17, 1999 I˙Zmit Bay (Turkey) Tsunami: Regional (Tectonic) and Local (Mass Instabilities) Causes Marine Geology 225 (2006) 311–330 www.elsevier.com/locate/margeo The generating mechanisms of the August 17, 1999 I˙zmit bay (Turkey) tsunami: Regional (tectonic) and local (mass instabilities) causes Stefano Tinti a,*, Alberto Armigliato a, Anna Manucci a, Gianluca Pagnoni a, Filippo Zaniboni a, Ahmet Cevdet Yalc¸iner b, Yildiz Altinok c a Universita` di Bologna, Dipartimento di Fisica, Settore di Geofisica, Viale Carlo Berti Pichat, 8, 40127 Bologna, Italy b Middle East Technical University, Department of Civil Engineering, Ocean Engineering Research Center, 06531, Ankara, Turkey c Istanbul University, Department of Geophysical Engineering, Division of Seismology, 34850, Avcilar, Istanbul, Turkey Received 23 November 2004; received in revised form 22 August 2005; accepted 2 September 2005 Abstract The MW =7.4 earthquake that affected the northwestern part of Turkey on August 17, 1999, and in particular the gulf of I˙zmit, had dramatic consequences also as regards tsunami generation. The main cause of the earthquake was a dextral strike-slip rupture that took place along different segments of the western part of the North Anatolian Fault (WNAF). The rupture process involved not only a number of distinct strike-slip fault segments, but also dip-slip ancillary faults, connecting the main transcurrent segments. The general picture was further complicated by the occurrence of subsidence and liquefaction phenomena, especially along the coasts of the I˙zmit bay and in the Sapanca Lake. Tsunami effects were observed and measured during post-event surveys in several places along both the northern and the southern coasts of the bay. The run-up heights in most places were reported to lie in the interval 1–3 m: but in the small town of Deg˘irmendere, where a local slump occurred carrying underwater buildings and gardens of the waterfront sector, eyewitnesses reported water waves higher than 15 m. The purpose of this work is to investigate on the causes of the tsunami by means of numerical simulations of the water waves. We show that the tsunami was a complex event consisting at least of the combination of a regional event due to tectonic causes and of a local event associated with the mass failure. As to the first, we are able to demonstrate that the observed tsunami cannot be explained only in terms of the sea bottom dislocation produced by the main right-lateral dislocation, but that the prominent contribution comes from the displacement associated with the secondary shallow normal faults. Furthermore, the large waves and effects seen in Deg˘irmendere can be explained as the consequence of the slump. By means of a stability analysis based on an original method making use of the limit equilibrium concept, we show that the slump was highly stable before the earthquake and that it was triggered by seismic waves. Simulation of the tsunami induced by the slump was carried out by a two-step numerical code that computes the landslide motion first, and then the generated water wave propagation. It is shown that the computed local tsunami matches the experimental data. D 2005 Elsevier B.V. All rights reserved. Keywords: 1999 Kocaeli earthquake; tsunami modeling; earthquake-induced tsunami; slump-induced tsunami * Corresponding author. Tel.: +39 051 2095025; fax: +39 051 2095058. E-mail address: [email protected] (S. Tinti). 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.09.010 312 S. Tinti et al. / Marine Geology 225 (2006) 311–330 1. Introduction the last catastrophic occurrences of a long series of disastrous and lethal natural events that hit this region. 1.1. Tectonic setting and earthquakes Here earthquake generation is governed chiefly by three main tectonic structures: the East Anatolian Fault The Anatolian peninsula is one of the most seismi- (EAF), the North Anatolian Fault (NAF) and the Hel- cally active areas in the world. The 1999 Kocaeli lenic Arc (Fig. 1). The left-lateral EAF is circa 550 km earthquake and the associated I˙zmit bay tsunami are long and represents the border between the Anatolian Fig. 1. Upper panel: Sketch of the Marmara Sea crossed by the Western section of the North Anatolian Fault (WNAF) system, which splits into three branches: northern, middle and southern branch. TB, Tekirdag˘ Basin; CMB, Central Marmara Basin; CB, C¸ inarcik Basin; IB, I˙zmit Bay; KP, Kapidag˘ Peninsula; GP, Gelibolu Peninsula. Lower Panel: The WNAF is part of the complex fault pattern resulting from the Anatolian Plate dynamics. KJ, KarlVova Junction; NB, Niksar Basin; TEB, Tas¸ova-Erbaa Basin; HLB, Havza-Ladik Basin. S. Tinti et al. / Marine Geology 225 (2006) 311–330 313 block and the Arabian plate. The NAF is a dextral some tens of kilometers to the south, passing through system of faults forming the broad arc boundary be- the Manyas and the Uluabat lakes and through the tween the Eurasian plate and the Anatolian block. It area of Bursa (Gu¨len et al., 2002). Historical earth- extends for about 1500 km from the northern Aegean quake activity seems to be more intense on the south- Sea, through the Marmara Sea up to the easternmost ern than on the middle branch of the WNAF. Several part of Turkey where it connects to the EAF at the large earthquakes are known to have occurred here in Karliova triple junction. Its western section, denoted the last three centuries. The most recent sequence of hereafter as WNAF, splays into two branches (Fig. 1), major quakes manifested as an eastward migration of approximately in correspondence of the Mudurnu Val- rupture starting with the October 6, 1944 (MS 6.8) ley fault, east to Akyazi (Barka and Kadinsky-Cade, Edremit-Ayvalik earthquake, with epicentre of 1988; Barka, 1992; Gu¨len et al., 2002). The northern 26.568E, 39.488N (earthquake database of Bosphorus one starts from the North Aegean Sea, trending NE University, Kandilli Observatory, and Earthquake Re- along the gulf of Saros; then it crosses west–east the search Institute (KOERI), available at http://www. central and northern section of the Marmara Sea where koeri.boun.edu.tr/jeofizik/defaulteng.htm), and con- it broadens in a wide complex of strike-slip and dip-slip tinuing with the March 18, 1953 (MS 7.2) Yenice- faults; next, it penetrates in the I˙zmit bay, whence it Go¨nen quake with epicentre of 27.368E, 39.998N continues towards the Sapanca Lake, further joining the (KOERI database), probably occurring on the middle main NAF system (Barka and Kadinsky-Cade, 1988). branch, and with the October, 06, 1964 (MS 6.9) A number of large historical earthquakes are associated Manyas-region shock: epicentre 26.238E, 40.308N with this branch. For example, fault rupture migrated (KOERI database). Hence the sequence affected the from east to west in the 18th century involving pro- overall sector from the Aegean Sea up to the Uluabat gressively all this system of faults from I˙zmit to S¸arko¨y, Lake (Hubert-Ferrari et al., 2000; Ambraseys, 2002a; first with the 1719 (I˙zmit, MS 7.4), and then with the Barka et al., 2002). 1754 (eastern Marmara, MS 6.8) and with the May (eastern Marmara, MS 7.1) and August (Ganos, MS 1.2. Transtensional tectonics and pull-apart basins 7.4) 1766 shocks (Ambraseys and Finkel, 1995; Ambraseys, 2002a). Notice that westward migration Due to the collision between the Arabian and Eur- of seismicity has been identified in several sequences asian plates, the Anatolian plate is squeezed and pushed of earthquakes also along the main section of the NAF westward moving along the NAF and EAF. The tec- and may be related to the basic long-term tectonic tonic style is predominantly transcurrent along the NAF processes of inter-plate interaction going on here system, where slip occurs along a series of faults tens of (Barka, 1992; Armijo et al., 1999; Hubert-Ferrari et kilometers long with prevailing EW trend. But at the al., 2002). Later, major quakes that are attributed to junction of several of such segments, due to their segments of this branch occurred in 1878, in 1894 (MS relative offset and overstepping, extensional tectonics 7.3) and in 1912 (MS 7.4) (Ambraseys and Finkel, occurs with the consequent formation of a sequence of 1991; Ambraseys, 2002a). The most recent occurrences pull-apart basins, such as the prominent depressions of are the 1943 (MS 6.4) Hedek-Adapazari earthquake and the Niksar, of the Tas¸ova-Erbaa and of the Havza-Ladik the 1963 (MS 6.4) C¸ inarcik earthquake. This is also the basins that one finds in the central portion of the NAF genetic place of the Kocaeli earthquake that is at the (Barka et al., 2000). In the WNAF, step-over depres- centre of our attention here, being the cause of the 1999 sions on land are often associated with lakes (e.g. the I˙zmit tsunami. Sapanca Lake in the northern branch of the WNAF, the The southern branch splits in turn into two further Iznik Lake in the middle strand and the Uluabat and the strands east to the Iznik Lake (Fig. 1), which makes Manyas lakes in the southern branch). Transtension and that the total number of the WNAF branches can be vertical movements are believed to be of foremost considered to be three: these further subsystems can importance especially on the northern branch of the be called the middle and the southern branch of the WNAF. Indeed the genesis of the Marmara Sea itself WNAF. The middle runs NE-ward from the Aegean is attributed to pull-apart processes (Hubert-Ferrari et Sea, south of the Dardanelles Straits, and intersects the al., 2002).
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